The present invention relates to X-ray equipment and, more particularly, to emissive coatings for targets of X-ray tubes.
X-ray tubes accelerate a beam of electrons through a vacuum to high electron velocity under a high electric field toward a metallic target. When the electrons are decelerated by impact with the target, a beam of X rays is emitted by the target.
Only about one percent of the electron energy produces X rays and the remainder is dissipated as heat. It is customary to aid the dissipation of heat by applying an emissive coating to the target.
One emissive coating is a ceramic layer consisting of zirconium, calcium and titanium dioxide. This coating is made by sintering a mixture of calcium oxide, zirconium dioxide and titanium dioxide to form a ceramic mass. The ceramic mass is ground and screened for a suitable range of particle sizes such as, for example, from about 10 to about 37 microns. The powder is applied to the target by conventional plasma spray techniques. Finally, the target, including the powder coating, is baked to fuse the powder to the surface and to outgas the target.
Modern X-ray targets employ molybdenum or alloy substrates. At temperatures exceeding about 1600 degrees C., they liberate carbon. The above conventional emissive coating powder requires a baking temperature of about 1640 degrees C. to produce a smooth, adherent coating. The liberated carbon, however, reacts with the coating at the interface to produce carbon dioxide gas thus disrupting adhesion. A poorly adhering coating may result.